pc64: Turn CLK_USE_* flags into tunable, skip code when results are unused.
* This should save ca. 1s during bootup, by skipping the i8254 and TSC
calibration based on the RTC, when we aren't actually using the result
in any way.
* For now, continue running that calibration code, when booting in verbose
mode, to keep printing these potentially useful calibration results.
* Setting the hw.calibrate_timers_with_rtc tunable to 1, will enable this
calibration code, and use those results for the i8254 and TSC frequencies.